Mounted Rack apps should have default named routes based on app name

This fixes a regression in 4.2.0 from 4.1.8.

https://github.com/rails/rails/pull/17823 fixed a similar regression regarding _explicitly_ named routes for a mounted Rack app, but there was another regression for the default value.

With a route like:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route'
    end

The "Prefix" column of rake routes gives the following:

- 4.1.8:         mountable_web
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     some_route   <- regression

This fixes the default to go back to being based off the name of the class like the docs specify: 785d04e310/actionpack/lib/action_dispatch/routing/mapper.rb (L558-L560)

Explicitly named routes still work correctly per https://github.com/rails/rails/pull/17823:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route', as: 'named'
    end

- 4.1.8:         named
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     named

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -579,15 +579,14 @@ module ActionDispatch
 
           raise "A rack application must be specified" unless path
 
-          rails_app = rails_app? app
+          options[:as] ||= app_name(app)
-          options[:as] ||= app.railtie_name if rails_app
 
           target_as       = name_for_action(options[:as], path)
           options[:via] ||= :all
 
           match(path, options.merge(:to => app, :anchor => false, :format => false))
 
-          define_generate_prefix(app, target_as) if rails_app
+          define_generate_prefix(app, target_as) if rails_app?(app)
           self
         end
 
@@ -612,6 +611,14 @@ module ActionDispatch
             app.is_a?(Class) && app < Rails::Railtie
           end
 
+          def app_name(app)
+            if rails_app?(app)
+              app.railtie_name
+            elsif class_name = app.try(:name)
+              ActiveSupport::Inflector.underscore(class_name).tr("/", "_")
+            end
+          end
+
           def define_generate_prefix(app, name)
             _route = @set.named_routes.get name
             _routes = @set

actionpack/test/dispatch/routing/inspector_test.rb

@@ -2,6 +2,11 @@ require 'abstract_unit'
 require 'rails/engine'
 require 'action_dispatch/routing/inspector'
 
+class MountedRackApp
+  def self.call(env)
+  end
+end
+
 module ActionDispatch
   module Routing
     class RoutesInspectorTest < ActiveSupport::TestCase
@@ -204,19 +209,36 @@ module ActionDispatch
         ], output
       end
 
-      class RackApp
-        def self.call(env)
-        end
-      end
-
       def test_rake_routes_shows_route_with_rack_app
         output = draw do
-          get 'foo/:id' => RackApp, :id => /[A-Z]\d{5}/
+          get 'foo/:id' => MountedRackApp, :id => /[A-Z]\d{5}/
         end
 
         assert_equal [
           "Prefix Verb URI Pattern        Controller#Action",
-          "       GET  /foo/:id(.:format) #{RackApp.name} {:id=>/[A-Z]\\d{5}/}"
+          "       GET  /foo/:id(.:format) MountedRackApp {:id=>/[A-Z]\\d{5}/}"
+        ], output
+      end
+
+      def test_rake_routes_shows_named_route_with_mounted_rack_app
+        output = draw do
+          mount MountedRackApp => '/foo'
+        end
+
+        assert_equal [
+          "          Prefix Verb URI Pattern Controller#Action",
+          "mounted_rack_app      /foo        MountedRackApp"
+        ], output
+      end
+
+      def test_rake_routes_shows_overridden_named_route_with_mounted_rack_app_with_name
+        output = draw do
+          mount MountedRackApp => '/foo', as: 'blog'
+        end
+
+        assert_equal [
+          "Prefix Verb URI Pattern Controller#Action",
+          "  blog      /foo        MountedRackApp"
         ], output
       end
 
@@ -229,21 +251,21 @@ module ActionDispatch
 
         output = draw do
           scope :constraint => constraint.new do
-            mount RackApp => '/foo'
+            mount MountedRackApp => '/foo'
           end
         end
 
         assert_equal [
-          "Prefix Verb URI Pattern Controller#Action",
+          "          Prefix Verb URI Pattern Controller#Action",
-          "   foo      /foo        #{RackApp.name} {:constraint=>( my custom constraint )}"
+          "mounted_rack_app      /foo        MountedRackApp {:constraint=>( my custom constraint )}"
         ], output
       end
 
       def test_rake_routes_dont_show_app_mounted_in_assets_prefix
         output = draw do
-          get '/sprockets' => RackApp
+          get '/sprockets' => MountedRackApp
         end
-        assert_no_match(/RackApp/, output.first)
+        assert_no_match(/MountedRackApp/, output.first)
         assert_no_match(/\/sprockets/, output.first)
       end